The present invention relates to a resistor composition for resistor-incorporated spark plugs, and more specifically to improvements in the resistance properties of the resistor, upon sealing, relying upon glass as a binder of the resistor.
Generally, conventional resistor compositions for resistor-incorporated spark plugs have been basically made up of inorganic components (referred to "base mixture" hereinafter), i.e., glass frit and inorganic aggregate, the aggregate substantially consisting of alumina, zircon, mullite, silica, clay, silicon nitride, boron nitride, aluminum nitride or the like or a mixture of these substances, and a carbonaceous material, the carbonaceous material encompassing carbon black, graphite or other organic substances capable of being carbonized by sealing/heating such as, for instance, glycerol, methyl cellulose, polyvinyl alcohol and the like. To further improve the stabilizing properties of the resistor with respect to resistance under load, one may use per 100 parts by weight of the aforesaid base mixture, 0 to 30 parts by weight of one or more components selected from the group consisting of oxides and carbides of rare earth elements and metals (Ti, Nb, Cr, etc.) of subgroups IVa, Va and VIa of the long period type periodic table (Iwanami Rikagaku Jiten, the third edition, pp. 1484-5), carbide of B or Si, and the like.
Borosilicate glass and barium borate glass are known in the art as glasses used to this end.
Such resistor compositions are charged in an insulator center bore of a spark plug, which is usually sealed to form a resistor body between a center electrode and a terminal rod, both being inserted in the center bore by means of a conductive sealing glass powder usually interposed between the resistor body and the center electrode or the terminal rod.
This resistor body has to be sealed so as to produce a given resistance value in order to prevent occurrence of radio frequency interference on ignition of the spark plug. However, conventional resistor compositions undergo variation in the resistance value depending upon the sealing temperature. Thus, it is difficult to obtain a desired constant resistance value in the spark plug, thus requiring stringent sealing temperature control for its preparation.